The normal processing operations to which thermoplastic polymers are subjected invariably involves high temperatures and these promote the deterioration of the polymers. The formulation of a polymer composition, i.e., one which contains the various stabilizing additives ordinarily required, usually is accomplished on a heated two-roll mill, or in a heated Banbury mixer, or both, and the temperatures at which such mixing steps are carried out are quite high, well above the temperature at which the polymer becomes fluid. At these temperatures the polymer will develop color, become brittle, etc., and such evidence of deterioration cannot be tolerated in the final product.
There are a number of heat stabilizing additives available which are effective to protect thermoplastic polymers from such deterioration. Among these are the organic phosphites. The dialkyl pentaerythritol diphosphites are especially effective for this purpose and are widely used. They are prepared by the reaction of two mols of an alcohol with diphenyl (or dichloro) pentaerythritol diphosphite. The reaction is illustrated by the equation: ##STR1## The product shown above is the spiro isomer, and where diphenyl pentaerythritol diphosphite is the reactant, as above, this spiro isomer comprises about half of the combined total of spiro and caged isomers. The caged isomer has the structure: ##STR2## When dichloro pentaerythritol diphosphite is substituted for the diphenyl compound as the reactant, however, the product which results is relatively pure spiro isomer. Inasmuch as the spiro isomer is itself more stable to hydrolysis, it is a preferred form.
Phosphite esters generally, however, are prone to hydrolysis, and their hydrolysis is accompanied by a corresponding loss of heat stabilizing effectiveness with respect to the polymer compositions in which they are used for that purpose. Moreover, such hydrolysis also frequently is accompanied by a tendency to blocking, i.e., a tendency for the ordinarily granular phosphite material to congeal into a single solid block.
Many attempts have been made to solve this problem. Additives such as triisopropanolamine have been found to impart a significant hydrolytic stability to dialkyl pentaerythritol diphosphites. The above-noted process for preparing relatively pure spiro isomer diminishes the problem. The phosphite can be stored in moisture-proof containers such as polyethylene bags until just before use. All of these have been very helpful, but they have not eliminated the problem, nor the desirability of a solution to the problem.